Over recent years, touch sensing devices have become more and more important. Touch sensing devices can be divided into three different categories: optical, resistive, and capacitive tracking devices.
Optical touch solutions are highly dependent on ambient lighting and the material of the tracking object. In addition, the separation of ‘touch’ and ‘pen’ inputs is not easily possible.
Resistive array-based sensors usually consist of two layers of conductive material, one with horizontal lines and one with vertical lines. When a user touches the surface, the horizontal and vertical lanes are alternately connected, enabling the flow of current. Although this approach is inexpensive and energy-efficient, the tracking resolution is limited to the space between the sensing lines. Alternatively, plane-type conductors with well-defined resistivity are used as top and bottom electrodes. The touch signal is measured by applying a voltage to one of the electrodes and detecting the resistance of the other electrode relative to the electrode edges. This is similar to treating the electrodes as a voltage divider. Here the resolution is mainly determined by the sensitivity of the read-out electronic, the separation between the electrodes (spacer) and the homogeneity of the electrodes' conductivity. However, the standard resistive touch panel concept is not suitable for pressure sensing.
From US 2009/0256817 A1 there is known a resistive, pressure-sensitive touch-based input device for tracking both touches as well as pens based on Interpolating Force Sensitive Resistance (IFSR). In this setup, the sensing material is mounted on a paper-thin flexible/bendable material and is able to sense pressure input.
Capacitive touch sensors consist of a thin conductive layer placed on an insulator, such as a glass. This layer serves as the electrode of a capacitor. A touch on the surface results in a distortion of the panel's electrostatic field and is measurable as a change in capacitance. However, capacitive sensing can only measure the touch location (resolution is limited by touch area). It is not suitable for pressure sensing. Another major disadvantage of this technique is that it relies on the dielectric properties of the human body; thus, stylus or objects cannot be tracked.
State-of-the-Art piezoelectric sensing devices exploit the piezoelectric effect only indirectly by detection of touch-induced surface (acoustic) waves via piezoelectric transducers placed at the device corners. Such devices are expensive due to the costs of the required inorganic piezoelectric materials and the involved costly assembling process. They provide only limited user interaction as for example the detection of a motion-less finger is impossible.
WO 2012/025412 A1 describes a method of producing piezoelectric and pyroelectric coatings.
U.S. Pat. No. 8,138,882 B2 describes the use of a sensing device in an “intelligent floor”.
Although some of the above mentioned sensor concepts provide multi-touch sensing capabilities, it is often not possible to track the pressure of the input efficiently and accurately. Also, it is desirable to track pen and touch operations separately. In addition, it is desirable to combine the tracking of pen and touch operations with pressure tracking.
The present invention aims to address these and other issues.